概述
在Android中，View的onMeasure()方法用来对控件进行测量，确定控件的宽高。该方法的两个参数widthMeasureSpec和heightMeasureSpec由父View计算后传入子view的measure()方法，再由子view的measure()方法传入onMeasure()方法，本文将介绍MeasureSpec的创建规则
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)LayoutParams
在开始源码分析前，我们要先介绍下LayoutParams，因为子view的MeasureSpec的创建需要用到子view的LayoutParams，LayoutParams有如下三种类型
- FILL_PARENT/MATCH_PARENT：填满父View
- WRAP_CONTENT：包裹内容
- 确定值：确定的值
源码分析
首先我们要先找到父View中调用子view的measure()方法的入口
首先先看View类的onMeasure方法如下，由于View是所有控件的基类，这里只是一个默认实现
我们应该看得是ViewGroup类型的类的onMeasure方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}查看源码后发现ViewGroup.java类中并未重写onMeasure()方法，且ViewGroup类是个抽象类，所以我们应该把目光投向ViewGroup的子类，比如AbsoluteLayout、LinearLayout、FrameLayout等
我们选其中一个看看，查看AbsoluteLayout的onMeasure()方法，可以知道如下的调用栈
可以看到measureChildren()和measureChild()是定义在ViewGroup中的，在子类中可以访问
AbsoluteLayout.onMeasure()->ViewGroup.measureChildren()->ViewGroup.measureChild()
接下来直接看measureChild()的源码
由下面的源码及其注释可以看到，子view的MeasureSpec是交由getChildMeasureSpec()方法来计算，终于是让我们找到了
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding.
* The heavy lifting is done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param parentHeightMeasureSpec The height requirements for this view
*/
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}接下来我们就看看getChildMeasureSpec()方法的源码，也是本文的重点内容
该方法的功能就是通过父view的MeasureSpec和子View的LayoutParams来计算出子View的MeasureSpec
父View的MeasureSpec有MeasureSpec.EXACTLY、MeasureSpec.AT_MOST和MeasureSpec.UNSPECIFIED三种类型，而子view的LayoutParams也有MATCH_PARENT、WRAP_CONTENT或者确定值三种类型，3*3得出9种情况，如下代码所示
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}总结
子view的MeasureSpec创建规则如下表所示
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